What about London (mid-April though...)? Plenty of good runners, you can get a championship start on 2:45, and there's more drink (water and sports) stations than you'll get just about anywhere else.
Otherwise, Rotterdam, but it's not as well organised on separating fast runners from slow.
I did Amsterdam marathon last year, There won't be many flatter than this one. Again it's a bit soon - 21st October. I'm not sure if you can still enter.